This invention relates to the removal of metal contaminants from hydroconversion catalysts. More particularly, metal contaminants are selectively removed from catalysts on refractory oxide supports by treatment with buffered oxalic acid solutions without dissolving the support.
Hydroconversion catalysts used to treat petroleum feedstocks become deactivated due to factors such as coke build-up and contamination by metals typically found in crude oil. It is known to use acids in the process of regenerating hydroconversion catalysts. U.S. Pat. No. 2,380,731 relates to the removal of iron and other metals from catalytic cracking catalysts by treating spent catalysts with organic acids and dilute mineral acids. Aqueous solutions of oxalic acid are preferred. U.S. Pat. No. 3,020,239 describes the removal of vanadium from molybdenum containing catalysts using aqueous glycolic acid solution. Other hydroxy acids and compounds similar to glycolic acid are unsatisfactory because of a concomitant removal of molybdenum. Concentration control is preferred to avoid leaching of aluminum from support material. U.S. Pat. No. 3,791,989 teaches the removal of vanadium from a hydroprocessing catalyst containing a Group VI or Group VIII metal by contacting the catalyst with an aqueous solution of oxalic acid before burning off any coke deposits. In example 1, fouled catalyst particles were boiled with concentrated oxalic acid solution with a preferential removal of vanadium over nickel or molybdenum. A reverse order is described in British Pat. No. 1,245,358 whereby deactivated Group VI or VIII catalyst on a carrier is first subjected to a coke burn-off followed by washing with aqueous oxalic acid solution having a concentration of 0.5M to saturation. Catalytic metal is not removed provided contact time with oxalic acid is limited.
In U.S. Pat. Nos. 4,089,806 and 4,122,000, hydro-desulfurization catalysts containing Groups VIB and/or VIII metals on a refractory support are regenerated using a combination of oxalic acid plus nitric acid and/or nitrate salts. The use of oxalic acid alone in environments severe enough to remove substantial amounts of vanadium is stated to result in dissolution of alumina support. Finally, according to U.S. Pat. No. 3,536,637, iron fouled ion exchange materials are rejuvenated by contacting fouled resin with oxalic acid.
While it is known that oxalic acid is useful for removing certain metal contaminants from hydroconversion catalysts supported on inorganic oxides such as alumina, oxalic acid suffers from the disadvantage of also removing or dissolving catalytic metals and support materials.